N=16Spherical Shell Closure inO24

The unbound excited states of the neutron drip-line isotope $^{24}\mathrm{O}$ have been investigated via the $^{24}\mathrm{O}(p,{p}^{\ensuremath{'}})^{23}\mathrm{O}+n$ reaction in inverse kinematics at a beam energy of $62\text{ }\text{ }\mathrm{MeV}/\mathrm{nucleon}$. The decay energy spectrum of $^{24}\mathrm{O}^{*}$ was reconstructed from the momenta of $^{23}\mathrm{O}$ and the neutron. The spin parity of the first excited state, observed at ${E}_{x}=4.65\ifmmode\pm\else\textpm\fi{}0.14\text{ }\text{ }\mathrm{MeV}$, was determined to be ${J}^{\ensuremath{\pi}}={2}^{+}$ from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter ${\ensuremath{\beta}}_{2}$ of the ${2}_{1}^{+}$ state was deduced, for the first time, to be $0.15\ifmmode\pm\else\textpm\fi{}0.04$. The relatively high excitation energy and small ${\ensuremath{\beta}}_{2}$ value are indicative of the $N=16$ shell closure in $^{24}\mathrm{O}$.